Electric power steering apparatuses for automobiles generally employ a speed reduction gear. In a column-type EPS (Electric Power Steering System), for example, the speed reduction gear transmits rotation of an electric motor from a small gear such as a worm to a large gear such as a worm wheel for reduction of a rotation speed and amplification of an output, and then applies the output to a column for torque assist of a steering operation.
For an electric power steering apparatus for use in a light four-wheel automobile or in a relatively small ordinary automobile, for example, it has been a common practice in recent years to use a resin as a material for at least one of the small and large gears of the speed reduction gear, preferably the large gear, with the aim of reducing a teeth striking sound for noise reduction, reducing the weight of the gear and reducing a sliding resistance. More specifically, the large gear is composed of a metal core and an annular gear body of a resin fitted around the metal core.
Exemplary resins widely used as a material for the gear body include polyamide resins such as MC (monomer casting) nylon, PA6, PA66 and PA46. In order to suppress dimensional change mainly attributable to moisture absorption, heat and the like for improvement of the dimensional stability of the gear, reinforcement fibers such as glass fibers are blended in the polyamide resin (see, for example, JP-2002-156025-A2).
Similarly, a less moisture-absorptive polyamide resin such as PA12, PA11, PA612, PA610, an aromatic polyamide or a modified PA12 is blended in the aforesaid polyamide resin in order to suppress dimensional change due to moisture absorption for improvement of the dimensional stability of the gear (see, for example, JP-2003-83423-A2).
For an electric power steering apparatus for use in a larger automobile, it has also been contemplated in recent years to use a resin as a material for the gear of the speed reduction gear.
In recent years, an attempt has been made to further reduce the size of the electric power steering apparatus or to reduce a space required for installation of the electric power steering apparatus in an automobile irrespective of the size of the automobile for enhancing fuel economy to cope with environmental issues or for improving a cabin comfort in the automobile.
A gear composed of PA6 which is the most popular and the most excellent in strength characteristics and the like among the polyamide resins, however, suffers from a considerable dimensional change due to moisture absorption. Even if the dimensional stability of the gear is improved by the addition of the glass fibers as described above, the gear is liable to be expanded during transport of the automobile for overseas export or due to aging, whereby the torque of the electric power steering apparatus changes. In addition, the gear composed of PA6 is insufficient in heat resistance.
It is also conceivable to form the gear by PA66 which is the most popular next to PA6 among the polyamide resins, less susceptible to dimensional change due to moisture absorption and excellent in heat resistance.
Where the gear of PA66 is applied to the electric power steering apparatus for the larger automobile or the smaller size electric power steering apparatus described above, the gear fails to fully satisfy a durability requirement for the speed reduction gear, resulting in breakage at a relatively early stage.
This is because, as the size of the automobile is increased, a greater electric motor output is required and a torque to be transmitted through the speed reduction gear in the electric power steering apparatus is increased.
Further, as the size of the electric power steering apparatus is reduced, it becomes more difficult to increase the module of the large gear of the speed reduction gear for reduction of the face pressure of the gear, and the face pressure to be transmitted from the small gear tends to be increased.
In order to improve the dimensional stability by adding the less moisture-absorptive polyamide resin to the polyamide base resin, the less moisture-absorptive polyamide resin should be blended in a great amount on the order of 10 to 40 wt % based on the total amount of these resins. Therefore, even if PA6 having a higher strength is used as the polyamide resin, the gear for use in the electric power steering apparatus having a higher torque or a smaller size is insufficient in strength, resulting in breakage at a relatively early stage.
These problems are associated not only with the speed reduction gears for the electric power steering apparatuses but also with ordinary speed reduction gears having a small gear and a large gear.